With the eMac coming to the masses, I see Apple phasing out the iMac (G3 chip) and only having the G3 processor inside the iBook for the foreseeable future. Eventually even the iBook will come to the G4 processor. At that point, I would hope that the G5 chips are inside Mac systems, at least in the pro line (PowerBook's and PowerMac's). We might see a half year of the G3 being inside Apple systems, but after the following expo, I would suspect they would be replaced with G4 processors.
Apple is still installing Both OS 9.2.x and OS X (10.1.x) on systems that are shipping, since people still need OS 9 for some applications and for classic mode (if they choose to not boot into 9).
Apple could have all three processors in systems, but it wouldn't be for very long (6 months or less). As for 'dual boot' on a Mac, it is many, many times better then the peecee world ever came up with. It also is still needed by enough people for Apple to make it easy for end users. I would be surprised if Apple removes OS 9 from systems before MWSF, if even then. We could see both OS's shipping on systems until all the major applications are for OS X (such as Quark XPress which is taking too long in MHO).